Conventionally, the on-vehicle radar device beam-forms a millimeter wave signal for target detection to thereby transmit it to a detection direction, and receives a reflected signal from the target (hereinafter, this signal is referred to as a “target-reflected signal”) to thereby detect the target in a detection region. Such a radar device includes a primary radiator that radiates a millimeter wave signal from a predetermined radiation surface, and a reflector that causes the millimeter wave signal radiated from the primary radiator to reflect to the detection direction, or that reflects the target-reflected signal from the detection region to thereby guide it to the primary radiator. Also, in the radar device, in order to detect a target having a predetermined width in the detection region, the beam of the millimeter wave signal must be caused to scan in a predetermined direction, e.g., in a horizontal direction. To this end, a conventional radar device uses an electronic scanning method for electrically scanning the beam of millimeter wave signal to be transmitted, employing a phased array antenna, or a mechanical scanning method for mechanically scanning the beam of millimeter wave signal to be transmitted by rotating/moving the primary radiator or reflector.
As shown in Patent Document 1, in the radar device using the conventional mechanical scanning method, the direction perpendicular to a beam transmission direction is made the direction of a rotating shaft of primary radiators, and primary radiators are arranged on the circumferential surface side of the rotating shaft, from the rotating shaft toward the outside thereof. Furthermore, a reflector is arranged over a predetermined angular range in a location spaced a predetermined distance apart from the rotating shaft. The shape of this reflector is designed so that a millimeter wave signal is transmitted within a desired scanning range. In such a radar device, by causing the signal radiated from the primary radiators to reflect by the reflector, a transmission beam having a directivity in a predetermined direction is formed. By rotating the primary radiator by the rotating shaft, the radiation direction of an electric wave radiated from the primary radiator is rotated, and the reflection direction of the electric wave is changed on the reflector, thereby forming the transmission beam with a predetermined intensity, for scanning within a predetermined angular range.
Patent Document 1: Japanese Patent No. 2693497
However, in an antenna device for the radar device using the conventional mechanical scanning method as shown in the Patent Document 1, for the purpose of forming a beam in the frontward direction of the antenna device, there exists the primary radiator between the reflector and the detection region in the front of the antenna, and simultaneously, the shape of the reflector is configured so as to correspond to a portion of not more than a semicircular surface of the side opposite to the frontward direction of the antenna device, in the circumferential surface that is centered on the primary radiator. As a result, when the primary radiator rotates up to the front side of the antenna device, i.e., up to the side where the reflector is absent, the millimeter wave signal radiated from the primary radiator is directly conveyed to the detection region without being beam-formed, and hence, the signal intensity is so low that this signal cannot be used as a signal for detection. As a consequence, the radiated signal while the primary radiator is not opposed to the reflector is wasted, resulting in reduction in the utilization efficiency, i.e., in radiation efficiency of the primary radiator. Also, since detection cannot be performed while the primary radiator does not face the reflector side, the detection efficiency is reduced.